George and Lennie are talking about their dream of living on a ranch and owning rabbits. Lennie wants to be the one that tends to the rabbits. Since they have nothing, this dream symbolizes their hope for a good future. George and Lennie focus on their goal to give them motivation to keep working hard.
Curley attacks Lennie because Lennie laughed when Slim made a joke about Curley. Lennie refused to hit Curley back at first but George yelled to Lennie to hit him back. Lennie grabs Curley's hand and breaks it. Lennie never wants to hurt anyone or upset George that is why he does not retaliate until George tells him too.
Curley's wife comes over to Lennie and starts speaking to him. She offers to let him touch her hair because she says it is soft. Lennie starts touching her hair but when she tells him to stop he won't. She starts screaming and fighting to get away and lennie starts grabbing on tighter until eventually he snaps her neck. Lennie ends up killing things he wants to touch, for example the mice. The more trouble Lennie gets in, the smaller the chance that George and him have at accomplishing their dream.
George finds Lennie after what happens and starts talking to him. George kills Lennie to end his suffering and to stop him from doing any more terrible things. Lennie continues to ruin their chances of ever being able to achieve their dream of living on the ranch and taking care of rabbits. George is very sympathetic when he kills Lennie, before he does it he talks to him about the future they wanted.